1. Field of Invention
The present invention is directed to an 18% Ni-Mo-Co maraging steel which possesses a substantially improved toughness. More particularly, the present invention relates to a method for processing an 18% Ni-Mo-Co maraging steel to obtain a product which possesses the best combination of tensile strength and toughness.
2. Description of the Prior Art
The alloy compositions of the commercial 18% Ni-Mo-Co maraging steels are designed to provide maximum toughness at strength levels of 200-350 ksi. They owe their unique properties to a tough ductile iron-nickel martensite structure which age hardens at approximately 900.degree. F. The primary precipitate responsible for the strengthening of the steel is Ni.sub.3 Mo. Cobalt enhances the effect of Ni.sub.3 Mo by decreasing the solubility of molybdenum in the matrix. Ni.sub.3 Ti also has been identified in maraging alloys, but it is believed to be a secondary precipitation hardening reaction product.
Strength of these materials is also increased by raising the titanium content in base compositions that are progressively slightly enriched in nickel, cobalt and molybdenum to maintain the optimum toughness. However, at and above a 300-ksi yield strength, the enriched compositions are subject to austenite retention at regions of alloy segregation.